A marking gauge has a fence that can be moved and locked
along a beam. There is a single pin to scribe one line. It is used primarily
for scribing tenons and dovetail shoulder lines, and occasionally to mark
out a rabbet. The pin is fixed and the fence slides to the correct location
on the beam and then is locked in place. There are dozens of mechanisms
for locking the fence on the beam, some using a captured wedge but more
likely some sort of thumbscrew.

A mortise gauge has 2 pins to scribe both sides of a mortise
simultaneously. Some mortise gauges, called combination mortise gauges,
have single pin on the other side so you can use it as a regular marking
gauge, and other types allow you to retract one pin into the fence, for
the same reason. If you have trouble getting both pins to scribe evenly,
check that both pins are at the same height. If they aren't, file one
down slightly. Depending on the design, the movable pin is adjusted by
means of a slider or a screw adjuster. The advantage of the screw adjuster
is that the movable pin will stay put, independent of later fence adjustments.
However, this convenience comes at a greater cost and complexity of mechanism.

(Note: we typically carry many different styles of the same gauge with
different features at different prices. The gauge illustrated and linked
at left is just one specific mortise gauge that we are using to illustrate
the genre.)

A cutting gauge is a marking gauge with a knife
instead of a pin. Basically, the theory behind this distinction is that
cutting across grain with a knife is better than using a pin. Since the
pin must be pretty sharp, I don't see much of a difference in use, although
I personally I prefer the pin. Nevertheless, a cutting gauge does come in
handy with delicate operations, such as dealing with veneers.

A panel gauge is basically a long marking gauge
with a beam that can go from a foot to several feet long, depending on the
application. A panel gauge's major use is in scribing boards to the correct
width. In the old days, before table saws and accurate rip fences, after
planing one edge of a board true you would use a panel gauge to scribe a
parallel line for the width of the board. Then you'd saw and plane to that
line. Panel gauges are also extremely handy for marking locations in the
center of a panel. These days in a modern shop they aren't used much, but
when you need one they can make layout much faster.

When you start out a good combination mortise gauge
is the most versatile tool to have. However, as you get involved in more
complicated projects, having a bunch of gauges whose settings you can maintain
for the duration of a project is very handy. Even though a combination gauge
can be used as both a mortise gauge and a marking gauge, I typically have
a mortise gauge set for the mortise and stock I'm using, and a different
marking gauge to hold the setting for the tenon length. This way I can go
back and forth with the identical settings without worrying about tiny differences
that can cause trouble. Once I set the gauge, I put masking tape over the
thumbscrew as a reminder so I know it's set.

To control the amount of pressure on the pin so that
you get a consistent scribe line, rest one edge of the entire length
of the beam on the work and then tilt the beam until the pin makes
contact with the work. Using the beam to control the amount of pressure
on the pin makes a world of difference. (Click on picture to
enlarge)

Don't run a gauge off the end or a board - this can
cause splintering. Instead you should stop scribing before you reach
the other end. At that point, you should start again from the other
edge and meet your line in the middle (at the same setting but from
the opposite direction).(Click on picture to enlarge)

If you have trouble seeing the scribed line, take
a very sharp pencil, place the point in the scribed line at a slight
angle to the wood and pull it along the scribed line. Let the scribe
guide the pencil. This will leave a little lead on both edges of
the scribe so you can see it better. For an even finer line, you
can go over the penciled (scribed) line with an eraser. What's left
will be a penciled line that is very precise (you've erased the
excess graphite) yet still easy to read.(Click on picture to
enlarge)

Marking and mortise gauges are some of the most used measuring
tools in the shop, and consequently we stock a fair selection.
One of the most important concepts in woodwork is the idea of
referencing measurements. It rarely matters what the exact measurement
is - 0.500" as opposed to 0.560"; the important thing is that
the measured parts are consistent, and the marked lines are truly
parallel. So gauges are used not to set specific lengths, but
rather to allow you to take measurement from one item and transfer
it exactly to another, without actually needing to know the exact
measurement. For example, the way to set a marking gauge for scribing
a line for dovetailing is to set the gauge on one piece, with
the fence on one side so that the pin just goes over the other
side. Then lock the gauge. This gives you a perfect line to scribe
- the exact width of the wood plus a little extra for planing
the dovetails flush.